<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <input type="text" id="num" placeholder="输入数据" />
    <button id="btn">计算长度</button>
    <input type="text" id="rea" placeholder="输入数据" />
    <script>
      function calc(a) {
        var arr = String(a);
        return arr.length;
      }

      btn.onclick = function () {
        var a = parseInt(num.value);
        rea.value = calc(a);
      };
    </script>
  </body>
</html>
